This invention relates to optical recording and reproducing apparatus employing media preformatted with wobbling guide grooves, more particularly to the detection of tracking error in such apparatus.
Optical reproducing apparatus is already widely manufactured and sold in the form of compact disc players, and apparatuses are now becoming available that can both reproduce and record information, using write-once-read-many media or overwritable media. In both types of apparatuses information is read from (or written on) spiral or concentric tracks by means of a laser beam. The beam is kept on-track by a system that detects tracking error, and a servo system that corrects the detected tracking error.
A variety of methods are used to detect tracking error. In the well-known push-pull method, the laser beam is reflected from the medium to a split photodetector. Signals from the two halves of the detector are added to obtain the reproduced information signal, and mutually subtracted to obtain a tracking error signal.
In another method known as the three-beam, triple-beam, tri-beam, or twin-spot method, three laser beams are focused onto the medium, creating three spots that are aligned at an angle to the tracks. The center spot is used for reading or writing information. The other two spots, referred to as satellite spots, are reflected to two separate photodetectors, and the difference between the photodetector output signals is used as the tracking error signal.
Still another method, known as the differential push-pull method, also employs three spots, but reflects each spot to a split photodetector that generates a difference signal. The tracking error signal is obtained by adding the two difference signals from the satellite spots and subtracting the difference signal from the center spot.
These three methods were originally developed for use with media having regular circular or spiral tracks, but there has been a recent movement toward media on which time tracks are formed in wobbling guide grooves. For example, standards for compact disc (CD) media have been proposed in which absolute address information and information for constant linear velocity control are encoded in time wobble.
A general problem arising with such media is that if the tracking error detection system detects the wobble, the tracking servo may attempt to track the wobble, thereby defeating the purpose of time wobble. In the push-pull method, for example, if the servo system tracks the wobble accurately, the information encoded in the wobble is lost. A similar problem occurs in the differential push-pull method when the spacing between the three spots is one-fourth the repeating period of the wobble.
A somewhat different problem can occur in the three-beam method. If the two satellite spots are separated by a distance equal to the repeating period of the wobble, the servo system tracks the wobble at the locations of the satellite spots, thereby doubling the amplitude of the wobble at the central read/write spot. In recording, this leads to poorly formed pits; in reproducing, it degrades the quality of the reproduced signal.
The wobble-tracking problem is related to the problem of tracking offset, which can be caused by tilting of the rotating medium or track-following movement of the objective lens. This problem occurs particularly when the push-pull method is used. Previous attempts to correct offset by means of sensors that detect media tilt and objective-lens movement have been unsatisfactory, in part because they increase the complexity and cost of the apparatus. Attempts to avoid the second type of offset by moving the optical system as a whole instead of just the objective lens also increase the cost of the apparatus, because a more powerful actuator is needed.